For a conventional antenna the polarization properties are substantially identical for spatial directions, at least within the main lobe of the antenna. This means for example that a sector antenna which is vertically polarized is substantially vertically polarized for all directions constituting the desired sector coverage.
It has become attractive to provide reconfigurable antenna systems, among other things in order to provide power efficient site installations. If for example an antenna system at a site is configured for three sector operation during busy hours with a high traffic load, it can be reconfigured for omni-directional (one sector) operation when traffic load is low. The purpose of performing a reconfiguration is to allow partial shut down base station equipment in order to save energy. FIGS. 1A and 1B very schematically illustrate the radiation patterns (main beams) and signals corresponding to an arrangement in which three different signals S1, S2, S3 are fed via separate transmitter chains to one sector antenna each, hence representing a first configuration state for high traffic load. After reconfiguration to a second state for low traffic load, the three transmitter chains are combined in that only one signal OS, e.g. from one of the transmitter chains in a distribution network DN, by means of power splitting is split into three identical signals OS1, OS2, OS3 which are fed to multiple antennas, for example the three antennas referred to above. These signals will then interact, resulting in coherent and/or non coherent beam-forming depending on antenna polarizations.
If the polarization for the electromagnetic fields of the antennas are non-orthogonal, an interaction between the non-orthogonal field components from the different antennas will result that depends on both amplitude and phase of the respective components, also called coherent beam-forming.
This means that the relative positions of the antennas and the effective signal path lengths from the power splitter DN to the antennas will have an impact on the resulting radiation pattern. If the radiated field components of the three identical, multiple signals are orthogonal, the power of the combined field of the three field components is the sum of the powers of the signals. This power addition is called non coherent beam-forming. Such a non coherent beam-forming results in a different combined radiation pattern as compared to coherent beam-forming. The magnitude of the combined radiation pattern for non-coherent beamforming is independent of the phase values of the signals, i.e. of the antenna positions and signal path lengths, which means that these two properties do not have to be considered during design and installation of an antenna system. The problem is that coherent beam-forming of identical signals from different antennas results in interaction in adjacent sectors, the effects of this interaction being particularly strong for directions in which the radiated power from two or more antennas are of similar magnitude, the effects of the interaction being difficult or impossible to predict without detailed knowledge of access point (site) geometry and phase characteristics of all components being part of the transmitter chains. It has been realized that such an interaction can be reduced by using different, preferably orthogonal polarizations in adjacent sectors. However, in order to be able to use orthogonal polarizations in adjacent sectors to avoid coherent beam-forming, there must be an even number of sectors, when the site is equipped with conventional sector antennas. The situation with a site installation having an odd number of sectors will be described with reference to FIG. 2C, which is a simplified top view of the antenna orientations and radiation pattern polarization states of the arrangement described above, for example in FIG. 1A, 2A.
With the site in a low traffic state configuration, for example realized as shown in FIG. 2B, the same signal OS is provided to all three antennas, here a vertically polarized antenna v1, a vertically polarized antenna v2 and a horizontally polarized antenna h1. Since the number of, conventional, antennas is odd, two adjacent sectors will have identical polarization, and thus coherent beam-forming takes place between signals transmitted from antennas v1 and v2. The coherent beam-forming will affect the resulting radiation patterns, here illustrated as signal vectors (amplitude and phase representation) being added, |sv1+sv2|2. For signals transmitted via v2 and h1 and v1 and h1 respectively the combined radiation pattern (field) is the result of adding the power of the respective radiated signals, |sv1|2+|sh1|2 and |sv2|2+|sh1|2, which means that there is no dependence on the positions of the antennas and the signal path lengths. The vector addition that occurs when signals with the same orientation of the electrical field, i.e., the same polarization interact results in large fluctuations in the resulting signal magnitude, especially near sector borders where the signals have about the same magnitudes. In which spatial directions the constructive/destructive combinations occur depends on the relative signal phase
To avoid coherent beam-forming, an installation could be provided which has an even number of sectors and in which, conventional, antennas with alternating polarizations are combined when reconfiguration takes place. However, there is always a risk that, when the site is reconfigured, signals are transmitted in adjacent sectors via antennas with the same polarization. This is so because typically there are many feeder cables and the reconfiguration (reconnection) may take place quite far from the antenna
It should also be borne in mind that antennas often are located on high masts which means that a physical verification of the cabling is difficult, and time consuming.